Electromechanical valve actuators for actuating cylinder valves of an engine have several system characteristics to overcome. First, valve landing during opening and closing of the valve can create noise and wear. Therefore, valve landing control is desired to reduce contact forces and thereby decrease wear and noise. However, in some prior art actuators, the rate of change of actuator magnetic force between an armature and a core with respect to changes in the airgap length (dF/dx) can be high when the air gap is small (e.g., at landing). As such, it can be difficult to accurately control the armature and/or seat landing velocity.
Second, opening and closing time of the valve can be greater than a desired value, e.g., 3 msec. In other words, due to limited force producing capability, the transition time of some previous systems may be too slow, and therefore result in reduced engine peak power.
Third, the power consumption of an electric valve actuation (EVA) system can have an impact on vehicle fuel economy, engine peak power, and the size/cost of the electrical power supply system. Therefore, reducing power consumption of the actuator, without sacrificing performance, can be advantageous.
One approach for designing an electromechanical valve actuator of an engine with a permanent magnet is described in JP 2002130510A. Various figures show what appears to be a permanent magnet located below coils having an adjacent air gap. The objective of this reference appears to be to increase the flux density in the core poles by making the permanent magnet width (“Wm” in FIG. 4) wider than the center pole width (“Di” in FIG. 4). The air gaps 39 by the two ends of the permanent magnet appear to be introduced to limit the leakage flux. Apparently, to further increase the flux density in the core pole, the permanent magnet cross section shape is changed from flat to V-shaped in FIG. 9.
Such a configuration therefore results in the bottom part of the center pole (Wm) being wider than the top part (Di) to accommodate the permanent magnet, which is placed below the coil. The inventors herein have recognized that these two features give rise to several disadvantages.
As a first example, such a configuration can result in increased coil resistance or actuator height requirements. In other words, to provide space for the permanent magnet, either the height of the actuator is increased (to compensate the loss of the space for the coil), or the resistance of the coil is higher if the height of the core is kept constant.
As a second example, the flux enhancing effect may also be limited by actuator height. In other words, the amount of space below the coils available for the permanent magnet is limited due to packaging constraints, for example. Therefore, while some flux enhancement may be possible, it comes at a cost of (as is limited by) height restrictions.
Other attempts have also been made to improve the actuator performance by using a permanent magnet. For example, U.S. Pat. No. 4,779,582 describes one such actuator. However, the inventors herein have also recognized that while such an approach may produce a low dF/dx, it still produces low magnetic force due to the magnetic strength limit of the permanent magnet material. Alternatively, other approaches, such as in U.S. application Ser. No. 10/249,328, assigned to the assignee of the present application, may increase the magnetic force, but may not reduce the dF/dx for armature and valve landing speed control.
In one example, at least some of the above disadvantages can be at least partially overcome by a valve actuator for actuating a valve in an internal combustion, wherein the valve actuator includes at least one electromagnet having a coil wound about a core, at least one permanent magnet disposed at least partially within the core, and an actuating member disposed adjacent to the electromagnet, wherein the actuating member is coupled to a pivot and is configured to be pivotally moved by activation of the electromagnet to effect at least one of an opening and a closing of the valve.
As such, various advantages can be achieved in some cases, such as decreased resistance, decreased transition times, and increased force output, while maintaining reduced dF/dx and dF/di (which can help valve landing control).